What term can be used to describe something that is burdensome?

The term "onerous" is highly appropriate to describe something that is burdensome because it specifically conveys a sense of task or obligation that is oppressive or heavy to bear. When something is described as onerous, it implies that it involves a significant amount of difficulty, hardship, or inconvenience. This aligns perfectly with the notion of a burden, as burdens are generally seen as weights that one must carry or deal with, often causing exhaustion or stress.

In contrast, the other terms do not carry the same weight of meaning related to burdensomeness. "Adversarial" refers to a situation characterized by conflict or opposition rather than the heaviness of a burden. "Untenable" describes something that cannot be maintained or defended, typically in an argument or position, without directly indicating that it is burdensome. "Composite" relates to something made up of various parts or elements and does not convey the idea of being a burden at all. Thus, "onerous" distinctly encapsulates the essence of something that is burdensome.